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The Sonoran Desert Border Region HERO consists of two watersheds, the Santa Cruz River and the San Pedro River, as well as the counties and municipalities predominantly situated in these watersheds. Both watersheds straddle the United States–Mexico border with their rivers flowing north from Sonora, Mexico into Arizona, United States. On the Arizona side, Santa Cruz and Cochise Counties reside mainly in these basins and rely on the groundwater sources within the basins. On the Sonoran side, there are five municipalities: Nogales and Santa Cruz in the Santa Cruz Basin, and Cananea, Naco, and Agua Prieta in the San Pedro Basin. Most of the population in this border region lives in two urban transborder communities: Nogales, Arizona and Nogales, Sonora, situated on the western side of the study area and together referred to as Ambos Nogales; and Douglas, Arizona and Agua Prieta, Sonora situated on the eastern side. A third transborder community, Naco, Arizona and Naco, Sonora, located just west of Douglas/Agua Prieta, is very small. Other settlements of significant size dot the region, including Sierra Vista, Rio Rico, Douglas, and Benson on the Arizona side, and Santa Cruz and Cananea on the Sonoran side (Figure 14.1).
The Sonoran Desert Border Region is semi-arid to arid, with summer temperatures frequently reaching over 104°F (40°C). The region experiences bimodal winter/summer precipitation patterns resulting from midlatitude frontal systems in winter and from thunderstorms within the regional North American monsoon circulation in summer (Adams and Comrie 1997; Sheppard et al. 2002).